City to tap into water purification

Amy Dorman, senior civil engineer with the city of San Diego's Public Utilities Department holds a sample of water purified to the quality level of pure distilled water at the Advanced Water Purification Facility in University City at the North City Water Reclamation Plant.
— Howard Lipin

Amy Dorman, senior civil engineer with the city of San Diego's Public Utilities Department holds a sample of water purified to the quality level of pure distilled water at the Advanced Water Purification Facility in University City at the North City Water Reclamation Plant.
— Howard Lipin

At the North City Water Reclamation Plant in San Diego, senior engineer Amy Dorman drew a flask of cool, colorless water from a tap.

The crystalline liquid had a murky past. It started as sewage, and then passed through a series of treatments that scrubbed it as pure as distilled water.

This water, produced at the city’s 1-million-gallon-per-day demonstration project at the North City facility, is the prototype for what may eventually become a major source of San Diego’s water. And it’s a new way of looking at wastewater for the arid West.

“It is very much cutting-edge,” said San Diego Public Utilities director Halla Razak. “There is a lot of excitement, especially with the drought, that this is a sustainable, locally controlled, drought-proof new water supply that is a great solution for the Southwest.”

With that potential in mind, San Diego plans to construct a water-purification plant that would produce 83 million gallons per day by 2035. At that point, purified water could provide about a third of San Diego’s supplies, Razak said.

Officials estimate the project will eventually cost nearly $2 billion. But that expense could be offset by eliminating the need for $1.8 billion in overdue upgrades to the Point Loma Wastewater Treatment Plant, she said. And it would help San Diego attain the water self-sufficiency that has long eluded the region.

At the North City plant, a demonstration project that was launched in 2011 has fine-tuned the steps that city officials will follow to produce purified water from wastewater. About 80 percent of wastewater is recycled by the end of the process.

It starts with micro-filtration: rows of plastic tubes containing bundles of tiny straws. Microscopic pores on their surfaces allow water molecules in, and then filter out microbes and other contaminants.

From there, the water flows to reverse-osmosis units, which force the water through filters at high pressure to screen out organic material, salts and other solids.

“This is really the workhorse of the whole treatment train,” Dorman said. “This is where 99 percent of what’s in the water is removed.”

It’s the same process used in desalination, but with less salt to remove, it takes only one-sixth of the energy required to treat seawater, Dorman said.

Water from each canister feeds into separate spigots, and workers test each source individually. So if one of the reverse osmosis units fails, they can quickly isolate the malfunctioning part without shutting down the entire system.

By the time the water runs through reverse osmosis, there are almost no impurities left in it. In the final step, workers run the water through a treatment that employs a combination of ultraviolet light and hydrogen peroxide to obliterate any potential contaminants.

“(The UV treatment) does disinfect 99.9999 percent of virus, protozoa and bacteria, but there isn’t expected to be anything in the water” after reverse osmosis, said water research manager Bill Pearce.

The technology isn’t new: Orange County and Los Angeles already use it, along with water districts in Colorado, Texas, Georgia, New Mexico, Arizona and Virginia, according to the Virginia-based Water Reuse Association.